1. Field of the Invention
The present invention relates to a device for measuring the degree of oxygen saturation in blood and more particularly to a noninvasive oximeter capable of using a digital arithmetic processing circuit.
2. Description of the Prior Art
Noninvasive oximeters are well known and capable of calculating the degree of oxygen saturation from a light transmission factor at a measuring point on the human body, for example, the tip of a finger when the measuring point is exposed to light.
In general, methods for measuring oxygen saturation in arterial blood without penetrating body tissue utilize the relative difference between the light absorption coefficient of hemoglobin (Hb) and that of the hemoglobin oxide (HbO.sub.2). The light absorption coefficient for Hb and HbO.sub.2 is characteristically tied to the wavelength of the light traveling through them. Both Hb and HbO.sub.2 transmit light having a wavelength in the infrared region to approximately the same degree. However, in the visible region, the light absorption coefficient for Hb is quite different from the light absorption coefficient of HbO.sub.2.
Prior art noninvasive photoelectric type oximeters (referred to as "oximeter" hereinafter) can utilize teachings of a photoelectric plethysmograph. Changes in the light transmission factor of a measuring point such as the tip of a finger are caused by changes in the amount of blood contained in the tip of the finger, namely, the pulse rate, which occur due to variations in the amount of the arterial blood in the tip of the finger. In order to discriminate between oxidized hemoglobin and reduced hemoglobin, the oximeter employs two lights of different wavelengths and the collected transmitted lights are subjected to photoelectric conversion and then logarithmic conversion. The light absorbencies of the tip of a finger with respect to these lights are evaluated, and the periodically varying components of the signal are picked up for an appropriate arithmetic operation to eventually calculate the degree of oxygen saturation in the blood. However, if outputs from the photoelectric conversion are processed through analog circuitry, then the outputs are susceptible to changes in power supply voltage, room temperature, etc., and bear a low S/N (signal to noise) ratio, thus requiring a compensation technique.
In contrast to analog processing, a digital arithmetic circuit can be expected to avoid the above discussed problems. Nevertheless, application of a digital arithmetic circuit to an oximeter results in the following practical problems. At a measuring point such as the finger tip, light is absorbed mostly by bones, skin or other connecting tissues; absorption by blood is much less and the alternating current component of the light absorbancy, indicative of the absorption by blood, accounts for only a few percent of the total measured signal. Information must be extracted from such a slight amount of alternating current component to calculate the degree of oxygen saturation. If it is desired to detect as small as 1% of change in the degree of oxygen saturation, then approximately 4% of change in the alternating current component compared to the carrier signal should be sensed, thereby requiring a sensitivity in the order of up to four significant figures in measuring the transmission factor at the tip of a finger or the like. While the current photoelectric conversion technique can satisfy such a sensitivity or accuracy requirement, it is undesirable to employ digital processing which needs an arithmetic operation circuit having at least a capacity of four decimal digits or ten binary digits.
Cited of general interest are U.S. Pat. Nos. 3,998,550; 3,948,248; 3,677,648 and "The Choroidal Eye Oximeter: Instrument for Measuring Oxygen Saturation of Choroidal Blood In Vivo" by Laing et al; IEEE Transactions on Biomedical Engineering, Vol. BME-22, No. 3, May, 1975, pg. 183.
The prior art is still seeking improved accurate oximeters that can be economically manufactured.